Multielement directional coupler



Jan. 10, 1967 J. K. HUNTCN 3,297,967

MULTIELEMENT DIRECTIONAL COUPLER Filed Oct. 2, 1964 DETECTOR -1 Fl l 1 l P24 LJ 1 SIGNAL [2 SOURCE LOAD I4J I6 2 Pi 4 5 a 3 5a 62 J o E .J m 52 3 E F1g 3 LL] 0:

68 INVENTOR ,,l JAMES KEITH HUNTON 1 O 1 BY e0 70 a0 I00 no 9 IN DEGREES ATTORNEY United States Patent 3,297,967 MULTltELEMENT DIRECTIONAL COUPLER James Keith Hunton, Los Altos, Calif., assignor to Alfred Electronics, Palo Alto, Calif., a corporation of California Filed Oct. 2, 1964, Ser. No. 401,144 14 Claims. (Cl. 333) This invention relates to RF signal translating devices and particularly to a directional coupler device for coupling a selected fixed portionof the wave energy available at the output terminal of its primary translation path to the output terminal of its secondary translation path.

In RF signal systems it is frequently desirable to maintain the wave energy, applied to a utilization device, constant and independent of the variation of signal strength supplied by the RF signal source. Since the RF signal from a source is subject to a number of. variations, such as for example changes in line voltages, modulation and other factors, a number of schemes have been used heretofore to level the power applied to the associated utilization device. One of the most frequently used schemes for achieving leveling action is to connect the utilization device to the signal source through a directional coupler which detects the transmitted signal.

The conventional directional coupler includes a primary and a secondary wave energy transmission path which are intercoupled to transfer a constant, predetermined and usually small portion of the wave energy flowing in the primary path to the secondary path. One end portion of the secondary path is connected to a detector means to measure the transferred energy which is directly proportional to the energy flowing through the primary path because the coupling is constant at a given frequency. The detector means may include control means for developing a signal commensurate with the wave energy. By suitably comparing the signal developed by the detector means with a suitable reference signal, a control signal may be developed which can be applied to the signal source to control its output power (or amplitude) in accordance with the reference signal.

The conventional directional coupler however is a narrow banded device and its dependence on frequency limits its usefulness for many applications. 'In systems where it is desirable to vary the frequency of the signal sources through a wide frequency band, the conventional directional coupler has been found wanting since the greater the frequency deviation from a mean operating frequency, the smaller is its response and accordingly the greater is the power it calls for the source to deliver to the utilization device.

It is therefore a primary object of this invention to provide a directional coupler device which is broadband in operation.

It is a further object of this invention to provide a directional coupler device for leveling the, output power from a signal source over a wide frequency band.

It is another object of this invention to provide a directional coupler device which is substantially independent of frequency.

It is still another object of this invention to provide a means and a method for compensating for the narrow frequency response characteristic of conventional direction-a1 couplers by removing from the primary transmission path an amount of wave energy which corresponds to the portion of the wave energy not transferred to the secondary transmission path because of the decreased response of the system at frequencies different from the mean operating frequency. In other words, compensation is achieved in this invention by removing the excess wave energy called for by the control signal because of "ice the decreased response of the coupler due to changes of the frequency from the mean frequency.

It is also an object of this invention to provide a method for utilizing directional coupling devices for broad band operations by which the microwave energy applied to a utilization device may be leveled accurately over a frequency range exceeding several octaves.

In accordance with one embodiment of this invention, a multi element directional coupler is provided which includes a primary and secondary transmission path intercoupled through a first coupling region having an electrical length equal to one-quarter of the mean wave length of the frequency band. The coupler further includes an additional transmission path intercoupled with the primary transmission path through a second coupling region having an electrical length equal to one-half of the mean wavelength of the frequency band.

The coupling coeflicient of the first coupling region is selected to provide a sampling signal in accordance with the requirement of the detecting and control element connected to the secondary transmission path for leveling action. The coupling coefficient of the second coupling region is selected in accordance with the teaching of this invention to remove sufficient energy from the primary translation path between the first coupling region and the load to compensation for the decreased response of the first coupling region when the frequency differs from the mean frequency which results in a control signal calling for more power generation than necessary for leveling action.

Other objects and a better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram of a microwave system showing the utilization of a directional coupler constructed in accordance with this invention for leveling the power applied to a load.

FIG. 2 is a schematic diagram showing the essential features of one embodiment of the multielement of this invention;

FIG. 3 shows a series of couplings versus frequency curves useful in explaining the operation of this invention; and

FIG. 4 is a schematic diagram of a further and more generalized multielement directional coupler constructed in accordance with this invention.

Referring now to FIG. 1 there is shown, in general schematic form, an RF system utilizing a multielement directional coupler for leveling or otherwise controlling the wave energy applied to a load. More particularly there is shown a directional coupler 10 which has a primary wave energy transmission path indicated by dotted line 12, a secondary wave energy path indicated by dotted line 14, and a further wave energy path indicated by dotted line 16. Primary path 12 is connected between a suitable signal source 18 and a utilization device 20 and for-ms the main transmission path of coupler 10.

Secondary wave energy path 14 has its output end coupled to a detector means 22 which provides a signal commensurate with wave energy received from path 14. The terms input and output end, as used herein, refer to the direction of primary flow of energy through the various paths in the absence of reflections. The detector signal may be utilized, in connection with some reference signal, to control the output power of signal source 18 in a manner well known to those skilled in the: art. Lead 24 is shown as applying the detector output signal, or the control signal, to source 18.

Referring now to FIG. 2 there is shown a multielement directional coupler consrtucted in accordance with the present invention. For simplicity, the reference characters in FIG. 1 have been utilized in connection with FIG. 2 where appropriate to designate like parts.

Secondary transmission path 14 is electromagnetically coupled to primary transmission path 12 through a coupling region designated by reference character 30. Coupling region 30 is inherently sensitive to the frequency of the wave energy being transmitted through path 12 and has an electrical length equal to one-quarter of the wavelength of the mid or center frequency of the operative frequency band.

As already explained, output terminal 32 of secondary transmission path 14 is connected to a sensing means such as a detector. The other end of path 14 is connected to an absorbtive load 34 which, without reflection, dissipates all wave energy flowing to the right side of the path (usually due to reflective wave energy moving upstream in path 12). Also, input terminal 36 and output terminal 38 of primary transmission path 12 are respectively coupled to the signal source and the utilization device.

Between coupling region 31) and output terminal 38, primary path 12 is coupled to wave energy transmission path 16 through a coupling region indicated by reference character 40. Like coupling region 30, coupling region 40 is inherently sensitive to the frequency of the wave energy transmitted through path 12 but its electrical length is selected to be equal to one-half of the wave length of the center frequency of the operative frequency band.

Transmission path 16, which will also be referred to as the compensating or correcting transmission path, has both its ends terminated by absorbtive loads 42 and 44 which, without reflection, dissipate all wave energy flowing in path 16. As will become clearer hereinafter, the primary purpose of path 16 is to remove the wave energy, transferred into it through coupling region 40, from primary path 12.

For optimum operation of the device shown in FIG. 2, the coupling region 30 and 40 are physically separate and region 40 is always downstream from region 30. The physical structure of each of paths 12, 14 and 16 may take any of the many well known forms such as strip lines, waveguides or coaxial conductors depending on the frequency of the wave energy, the power to be transmitted, and other factors not relating to present invention. Likewise coupling regions 30 and 40 may be probe, hole or slot coupled, the particular mode of coupling depending on the particular embodiment of the transmission path, and form no part of the present invention. Just by way of example, the method of coupling described in US. Letters Patent 3,121,848 issued to F. W. Kruse, Jr., et al. on February 18, 1964, for Continuously Variable Micro Strip Attenuator Using Directional Coupler is entirely satisfactory if variable coupling is desired.

Referring now to the operation of the device of FIG. 2 it is well known that the amount of energy coupled from one to another transmission path connected to one another by a coupling region depends on the coupling factor of the coupling region which will be designated by the symbol k, and on the frequency of energy to be coupled. It is further well known that a coupling region having an electrical length corresponding to an odd number of one-quarter wavelengths provides maximum coupling and a coupling region having an electrical length corresponding to an even number of one-quarter wavelength provides zero coupling.

As the frequency of the wave energy changes from the center frequency of the frequency band, the amount of wave energy coupled by coupling region 30 to secondary path 14 will diminish because its electrical length is no longer equal to the one-quarter wavelength necessary for maximum coupling. At the same time the electrical length of coupling region 40 is no longer one-half wavelength so that a certain amount of wave energy will he coupled from primary path 12 to corrective path 16 for absorption.

Accordingly, as the frequency of the wave energy applied to the load changes from the center frequency, the amount of wave energy coupled to detecting means 22 decreases so that detecting means 22 no longer provides a true measure of the wave energy propagated through path 12. Instead the indication will be lower so that the control signal which levels will cause source 18 to increase its output. At the same time coupling region 40 removes wave energy from path 12 and therefore diminishes the amount of wave energy available at output terminal 38.

By selecting a proper coupling factor for coupling region 40, the amount of wave energy removed from path 12 can be made substantially equal to the excess of Wave energy called for by detector means 22 so that the wave energy actually available at output terminal 38 is directly proportional to the wave energy coupled through coupling region 30 into secondary path 14.

Assume that the coupling factor wave amplitude (voltage) for coupling region 30 is k and for coupling region 40 is k and further that the input is unity. Then the wave amplitude coupled by coupling region 30, C is given by the expression:

0 jlc sin 6 The Wave amplitude transmitted along path 12 past coupling region 30, T is then equal to the wave amplitude applied to input terminal 36 (unity) less the amount of wave amplitude coupled to path 14 which is given by the expression:

/1k cos 0+j sin 0 2 The wave amplitude transmitted along path 12 past coupling region 40, T is equal to T less the wave amplitude coupled to path 16 which is given by the expression:

The power sampling ratio is the square of the magnitude of the ratio of the wave amplitude applied to detector 22 and the wave amplitude available at output terminal 38, which is found by dividing expression (1) by (3) and is given by the expression:

Tz T

The fiattest response is obtained by causing this ratio to be unity when 0 has a value corresponding to the edge of the frequency hand. Then the sampling ratio has the same value at the midband frequency as it has at the band edge frequency. For example, for a coupler to have the flattest response over a frequency band of one octave, that is from 60 to electrical degrees the value of 0=60 is inserted into expression (6).

so that k =%=.3077 and k =.5547

Expression (6) may be solved to provide the proper coupling factor k for any frequency bandwidth to assure that the ratio of the wave energy coupled to the secondary path to the wave energy applied to the utilization device is the same for the midfrequency and the high and low edge of the frequency band. What happens for intermediate frequency values can best be seen from FIG. 3 which provides a qualitative analysis.

Referring now to FIG. 3, curves 5t) and 52 respectively show the amount of wave energy, in db of the wave energy applied to the input terminal of the primary path, removed by coupling regions 3 and 40 versus frequency. The abscissa represents the frequency in terms of 6 which is the wavelength in electrical degrees. Base line 54 represents maximum coupling through region 30 and base line 56 represents zero coupling through coupling region 40. Lines 60, 58 and 62 respectively represent the low edge, center, and high edge of the frequency band.

From curve Sll it is readily seen that the amount of energy coupled through region 30 falls off as the frequency changes from the midfrequency. From curve 52 it is seen that the wave energy coupled through region 40 increases as the frequency approaches the edges of the frequency band. By selecting the relative ordinate scales of curves t and 52 such that each curve intercepts line 62 an equal distance below and above its base line respectively, as shown by 64 and 66, all intermediate points can be plotted. Curve 68 is such a constructed plot in which the intercepted distance it? is decreased by intercepted distance 68 to obtain intercepted distance 72 and therefore represents the coupler response over the entire selected frequency band. As is immediately seen, the response is substantially fiat over the whole band and will be within 0.1 db.

The flatness for the multi element coupler of this invention can be further increased by adding additional compensating elements as shown in FIG. 4. In this embodiment, the multielement direction coupler 80 includes aprimary transmission path 82 for connection between a source and a utilization device, a secondary transmission path 84 for applying a sampled portion of the wave energy to a detector means and a plurality of compensating transmission paths 86 88. Transmission path 84 88 are coupled to primary transmission path 82 through coupling regions 94 92, 914 respectively.

The compensating transmission paths have both end terminals terminated in a dissipative load to fully absorb the energy removed from the primary transmission path 82. The electrical length of the coupling regions associated with the various compensating transmission path are equal to n one-quarter Wavelengths of the center frequency where n is an integer greater than 1. By proper selection the coupling coefiicient of the various coupling region 92 94, the response of coupler 80 may be made as flat as desired and may span a frequency band of several octaves.

There has been described a directional coupler of multi element construction which is substantially fiat in its response over a large frequency band. By proper selection of the electrical length and the coupling factors of the various coupling regions connecting the primary transmission paths to the compensating transmission paths, the response of the directional coupler may be made as flat as desired and to expand over a frequency band of many octaves.

What is claimed is:

1. A directional coupler comprising:

a first wave energy translation path;

a second wave energy translation path;

a first coupling means for coupling said second to said first translation path, said first coupling means having an electrical length corresponding to onequarter wavelength of the mean operating wavelength of the wave energy transmitted through said first translation path;

at least one further wave energy translation path; and

a further coupling means associated with each of said further translation paths for coupling such further translation paths to said first translation path, said further coupling means having electrical lengths corresponding to (n+1) one-quarter wavelength of the said mean poe rating wavelength where n is an odd integer.

2. A directional coupler comprising:

a first wave energy translation path;

a second wave energy translation path;

a first coupling means for coupling said second to said first translation path, said first coupling means hav ing an electrical length corresponding to one-quarter wavelength of the mean operating wavelength of the wave energy transmitted through said first translation path;

at least one further wave energy translation path; and

a further coupling means associated with each of said further translation paths for coupling such further translation paths to said first translation path downstream from said first coupling means, said further coupling means having electrical lengths corresponding to (n+1) one-quarter wavelength of the said mean operating wavelength where n is an odd integer, the coupling coefiicient of each of said further cou pling means being selected in accordance with the band width of the frequency transmitted through said first translation path.

3. A directional coupler comprising:

a first wave energy translation path connected to a utilization device;

a second Wave energy translation path;

a first coupling means for coupling said second to said first translation path, said first coupling means having an electrical length corresponding to onequarter wavelength of the mid frequency of the wave energy band transmitted through said first translation path;

a compensating wave energy translation path; and

further coupling means for coupling said compensating translation path to said first translation path between said first coupling means and said utilization device, said further coupling means having electrical lengths corresponding to one-half wavelength of the said mid frequency.

4. A directional coupler in accordance with claim 3 in which the coupling coefiicient of said further coupling means is selected to remove an amount of wave energy from said first translation path which corresponds to the decrease in wave energy coupled to said second translation path through said first coupling means when the wavelength of the wave energy transmitted through said first translation path is equal to the upper or the lower edge of the frequency band transmitted.

5. In a directional coupler which includes a primary wave energy translation path for coupling a utilization device to a source of wave energy, and a secondary wave energy translation path coupled to a "wave amplitude detecting means, and in which the secondary translation path is coupled to said primary translation path through a first coupling means having an electrical length corresponding to one-quarter of the mean operating wavelength of a wave energy band transmitted through the primary translation path, the improvement comprising:

a further Wave energy translation path; and

second coup-ling means for coupling said further translation path to said primary translation path at a point between the first coupling means and the utilization device, said second coupling means having an electrical length corresponding to one-half of the mean operating wavelength.

6. In a directional coupler which includes a primary wave energy translation path for coupling a utilization device to a source of wave energy, and a secondary wave energy translation path for coupling to a detecting means, and in which the secondary translation path is coupled to said primary translation path through a first coupling means having an electrical length corresponding to one-quarter of the mean operating wavelength of the wave energy band transmitted through the primary translation path, the improvement comprising:

a further wave energy translation path; and

second coupling means for coupling said further translation path to said primary translation path at a point between the first coupling means and the utilization device, said second coupling means having an electrical length'corresponding to one-half of the mean operating wavelength and a coupling coefficient selected to transfer an amount of wave energy to said further translation path which is equal in amplitude, at the high or low edge of the wave energy band, to compensate for the decrease in frequency response of said first coupling means so that the detecting means provides a true indication of the wave energy applied to the utilization device.

7. In a directional coupler according to claim 6 in which at least one output terminal of said further translation path is terminated in an absorbtive load.

8. In a directional coupler which includes a primary Wave energy translation path for coupling a utilization device to a source of wave energy, and a secondary wave energy translation path for coupling to an amplitude detecting means, and in which the secondary translation path is coupled to said primary translation path through a first coupling means having an electrical length corresponding to one-quarter of the mean operating wavelength of the wave energy band transmitted through the primary translation path, the improvement comprising:

second coupling means for coupling wave energy from said primary translation path at a point between the first coupling means and the utilization device, said second coupling means having an electrical length corresponding to one-half of the mean operating wavelength and having a coupling coefiicient selected in accordance with the width of the wave energy band transmitted through said primary translation path; and

means coupled to said second coupling means for refiectionless reception of wave energy coupled out of said primary translation path.

9. In a directional coupler constructed in accordance with claim 8 in which the coupling coeflicient k of said second coupling means is given by the relation where .9:90)\0/)\, k is the mean operating wavelength of said selected frequency band, and A is the wavelength at either the upper or the lower edge of the wave energy band.

10. A multielement directional coupler comprising: a primary wave energy translation path for coupling a utilization device to a source of RF wave energy; a secondary wave energy translation path for coupling to a wave energy amplitude detection means; first coupling means having an effective electrical length of one-quarter of the mean operating wavelength for coupling said secondary translation path to said primary translation path; second coupling means included in said primary translation path and intermediate said first coupling 8 means and said utilization device, said second coupling means having an effective electrical length of one-half of the mean operating wavelength; and receiving the wave energy coupled out of said primary translation path by said second coupling means.

11. A multielement directional coupler comprising:

a primary wave energy translation path for coupling a utilization device to a source of RF wave energy adapted to provide wave energy of a selected frequency band;

a secondary wave energy translation path for coupling to a wave energy amplitude detection means;

first coupling means having an effective electrical length of onequarter of the mean operating wave length of said frequency band for coupling said secondary translation path to said primary translation path;

second coupling means included in said primary translation path immediate said first coupling means and said utilization device, said second coupling means having an effective electrical length of one-half of the mean operating wavelength of said frequency band, and a coupling coefiicient k given by the expression (1k )=Sll'l 0(1-k COS 2t9) where 0:90f0/A, X0 is the mean operating wavelenth of said frequency band, and A is the wavelength at either the upper or the lower edge of said frequency band; and

means associated with said second coupling means for receiving the wave energy coupled out of said primary translation path by said second coupling means.

12. In a RF system utilizing a directional coupler for measuring the amplitude of the wave energy applied to a utilization device, means for compensating for the decrease in coupler response as the wave energy frequency decreases or increases from the mean operating frequency towards the high or low edge of a preselected frequency band comprising:

means for removing an amount of wave energy from the wave energy translation path between said coupler and said utilization device which is substantially equal to the decrease of the wave energy coupled by said directional coupler as the frequency changes from said means operating frequency to the high or low edge of said frequency band multiplied by the reciprocal of the square root of the power sampling ratio of the coupler.

13. In a RF system utilizing a directional coupler for measuring the amplitude of the wave energy applied to a utilization device through a translation path, means for compensating for the decrease in coupler response as the wave energy frequency decreases or increases from the mean operating frequency towards the high or low edge of a preselected frequency band comprising:

means for removing an amount of wave energy from the portion of the wave energy translation path lying between said coupler and said utilization device so that the wave energy in the translation path is decreased by an amount sufficient to compensate for the lack of response of the directional coupler.

14. A device for obtaining a true indication of the wave energy delivered through a primary wave energy translation path to a utilization device which is substantially independent of the wave energy frequency within a selected frequency band, said device comprising in combination:

a secondary wave energy translation path;

a compensating wave energy translation path;

first coupling means for coupling said primary translation path to said secondary translation path and having an effective electrical length of one-quarter of the midfrequency wavelength of said selected frequency for frequencies of the Wave energy between said band; midfrequency and the edges of the selected frequency second coupling means for coupling said primary transband; and

lation path to said compensating translation path at indicator means responsive to the Wave energy coupled a point intermediate said first coupling means and 5 to said secondary translation path to provide the desaid utilization device, said second coupling region sired indication of the amplitude of the Wave energy having an effective electrical length of one-half of delivered to the utilization device.

the midfrequency wavelength of said selected frequency band and a coupling coefiicient selected such that the amount of Wave energy coupled to said 10 HERMAN KARL SAALBACH, Primary Examiner. compensating translation path, within said selected ELI LIEBERMAN Examiner 7 frequency band, offsets the decrease of the wave energy coupled to said secondary translation path NUSSBAUM, Assistant Examiner- No references cited. 

2. A DIRECTIONAL COUPLER COMPRISING: A FIRST WAVE ENERGY TRANSLATION PATH; A SECOND WAVE ENERGY TRANSLATION PATH; A FIRST COUPLING MEANS FOR COUPLING SAID SECOND TO SAID FIRST TRANSLATION PATH, SAID FIRST COUPLING MEANS HAVING AN ELECTRICAL LENGTH CORRESPONDING TO ONE-QUARTER WAVELENGTH OF THE MEAN OPERATING WAVELENGTH OF THE WAVE ENERGY TRANSMITTED THROUGH SAID FIRST TRANSLATION PATH; AT LEAST ONE FURTHER WAVE ENERGY TRANSLATION PATH; AND A FURTHER COUPLING MEANS ASSOCIATED WITH EACH OF SAID FURTHER TRANSLATION PATHS FOR COUPLING SUCH FURTHER 